1. Field of the Invention
This invention relates to a strand for transmitting an electrical or optical signal, and more particularly, to a composite strand wherein a strand for transmitting an electrical or optical signal is wound around a support or core strand consisting of a center strand of an elastic thread and a non-elastic thread wound around the center strand such that the composite strand is restricted for tension in certain directions by virtue of the non-elastic thread so as to prevent its breakage due to external forces, such as tension and shock.
2. Description of the Prior Art
Heretofore, signal transmitting strands, particularly, those making up a telephone cable have been composed of a copper wire material and a covering. Recently, a new material, such as an optical fiber, has been used.
In order to reduce material costs, these kind of cables use a unit strand made of signal transmitting wire having as fine diameter or thickness as possible. Such cables are buried underground or used as serial cables and are liable to be exposed to various external forces such as tension, shock and so on. Consequently, when the extent of the stress caused by such external force is greater than the breakage stress of the signal transmitting strand, a part or whole of the strand will break. This necessitated use of a thicker signal transmitting strand.
In view of these problems, signal transmitting strand made of composite strand as shown in FIG. 5 provides a strand made of a flat copper wire is wound around a center strand composed of synthetic resinous fiber in order to strengthen the strand for tensioning.
However, since the core strand is substantially non-extensible, applied tension force is also directly exerted to the flat copper strand, accordingly, if the center strand breaks it also results in breakage of the flat copper strand.
A copper wire constituting a strand making up a telephone cable should be thin to eliminate excessive material costs. This thinness limits its strength to withstand an applied tension force. If an amount of tension close to the limit of breaking stress is applied repeatedly, the signal transmitting strand which constitutes the composite strand will fatigue thereby impairing its endurance.
Such composite signal transmitting strands are not always gathered to form a cable, but also may be used as an individual signal place strands wherein the length must be arranged in a very exact manner. However, often it is cut somewhat shorter than that required due to errors in cutting, and thus will become unusable.